The invention relates to a method of preparing a luminescent Eu.sup.2+ activated strontium aluminate defined by the formula Sr.sub.4 Al.sub.14 O.sub.25 :Eu.sup.2+, in which a mixture comprising SrO, Al.sub.2 O.sub.3 and Eu.sub.2 O.sub.3 or compounds yielding these oxides upon an increase of temperature is heated in a weakly reducing atmosphere at a temperature of 1100.degree. to 1500.degree. C., and in which a flux is added to the mixture. The invention also relates to a luminescent strontium aluminate obtained in accordance with said method and to a low-pressure mercury vapour discharge lamp provided with a luminescent screen comprising such a luminescent strontium aluminate.
Luminescent aluminates defined by the formula Sr.sub.4 Al.sub.14 O.sub.25 :Eu.sup.2+ with an Eu-content of 0.1 to 10 mol % with respect to the Sr present, are known from Netherlands Patent Application 8201943. These aluminates are crystalline compounds having a characteristic X-ray powder diagram which is shown in the said Netherlands Patent Application. Although these aluminates can be obtained by starting from Sr/Al ratios of 1/2 to 1/5, it is assumed that the crystal phase substantially has the composition Sr.sub.4 Al.sub.14 O.sub.25. The said Netherlands Patent Application describes a method of preparation in which the aluminate is formed by a solid state reaction at a high temperature of a mixture comprising SrO, Al.sub.2 O.sub.3 and Eu.sub.2 O.sub.3. In this method the use of a flux or melting salt is very desirable. The fluxes H.sub.3 BO.sub.3, B.sub.2 O.sub.3 and SrF.sub.2 are mentioned as examples. After cooling, the product obtained can be washed with water in order to remove the greater part of the flux residues present.
The final product obtained in accordance with the known method still comprises small quantities of flux residues after washing. The presence of these residues may have a detrimental influence on the Eu.sup.2+ emission. It also appears to be difficult to obtain the desired crystal phase free from disturbing side-phases. A great drawback of the known method is that the aluminate obtained is poorly crystallized and has an unfavourable grain size and grain size distribution so that the luminous flux obtained is not optimum.
The said Netherlands Patent Application states that the aluminate may comprise small quantities of barium (up to at most 10 mol %). The presence of such small quantities of barium does not, however, yield any advantages and is generally found to decrease the efficiency.
An object of the invention is to provide a method of preparing luminescent strontium aluminate defined by the formula Sr.sub.4 Al.sub.14 O.sub.25 :Eu.sup.2+ in such a way that the desired crystal lattice of the aluminate is formed very satisfactorily so that the aluminates have a very efficient Eu.sup.2+ emission.